Synopsis

7T imaging of neurovasculature with TOF affords sub-millimeter
resolution and can make it possible to visualize extremely small
vessels such as the lenticulostriate arteries; however, TOF can be hampered by
flow and pulsation artifacts. K-space reordering and the administration of a
contrast agent can greatly reduce these artifacts while still allowing for
small vessel conspicuity. We acquired gadolinium and ferumoxytol-based MRA
alongside TOF in three patients and two radiologists rated vessel conspicuity
and artifact. Artifacts were diminished with contrast, though there were no
differences in artifact due to k-space reordering, and vessel conspicuity was
high in all acquisitions.

Purpose

Recent reports have investigated the use of time-of-flight (TOF) magnetic
resonance angiography (MRA) at 7 Tesla and found that the resolution afforded
by ultra high field imaging relative to 1.5 or 3T increased conspicuity of
small branching vessels while having
little to no effect on SNR in large vessels1. T1 weighting in vasculature on an MPRAGE sequence was
compared between non-contrast and contrast-enhanced protocols at 7T and found
improved assessment of small peripheral segments with contrast2. However, intracranial contrast-enhanced
(CE) MRA has yet to be reported when compared to TOF at 7T.

Methods

3 patients were entered into an OHSU IRB-approved
3 day protocol and scanned on a MAGNETOM 7T (Siemens) with a quadrature transmit 24-channel recieve head
coil (Nova Medical). The first day contained TOF (TR/TE/FA=15ms/5.58ms/20°, 5
slabs of 50 0.4mm slices, FOV=160x200mm, matrix=618x768, 0.35x0.26x0.69mm3
voxels), and CE-MRA (TR/TE/FA=7.09/2.3/12°,
208 0.4mm slices, FOV=165x200mm, matrix=416x512, (0.4mm)3 voxels) immediately
after 0.2mmol/kg gadoteridol (Gd) administration. CE-MRA with 4mg/kg
ferumoxytol (Fe) was performed on a second day. CE-MRA was acquired with linear
k-space ordering and with 3D centric reordering for assessment of pulsation and
ghosting artifacts.
CNR and
SNR measurements were taken in the axial plane through the ACA spanning two
hemispheres and through a cortical M4 segment (Figure 1) for all sequences; two
blinded radiologists rated for a) visualization of
dural venous sinus (DVS), b) visualization of M1/A1 segment (M1A1), c)
visualization of M4/subcortical arteries (M4), d) visualization of
lenticulostriate vessels (LSV), and e) for the presence of flow and pulsation
artifacts (FPA) in the phase encoding axis in the axial plane at the ACA. Ratings
for visualization were 1 – 5: visualization without question, visualized but
artifact impacts evaluation, visualized with less than 90% certainty,
visualized less than 50% certainty, and not visualized, respectively. Artifacts
were rated 1-5: no artifact, artifact present but does not affect vessel
conspicuity, artifact present with minor effect on vessel conspicuity, artifact
present with major effect on conspicuity, and artifact renders vessel
unreadable. Maximum intensity projections were calculated through the slice
direction between the circle of Willis and the top of the lateral ventricles
for qualitative comparison (Figure 3).

Results

There was no significant
difference in SNR or CNR between any of the scan types in the ACA (Figure 1) or
in a cortical M4 segment (Figure 2), though SNR and CNR varied widely across
patients. Raters were in modest agreement for LSV and M4 (r = 0.28 and 0.20,
respectively), good agreement on DVS (r = 0.62) and excellent agreement on M1A1
(r = 0.95) and artifacts (r = 0.95). Ratings for visualization (Figure 3) of small
vessels (LSV and M4) were best with TOF (1.8 and 2.2) and Gd (2.0 and 1.9),
though vessels were visualized with Fe (3.3 and 3.0). M1A1 was rated most
highly in Gd (1.5) and TOF (1.8), with Fe (2.8) also providing good visualization.
Finally, DVS was rated highest for Fe (1.5), with Gd (2.6) and TOF (2.0)
providing excellent visualization. Flow and pulsatile artifacts were largest in
TOF (2.7) and had a minor effect on conspicuity; artifacts were the same with linear ordering and 3D centric reordering (1.7 and 2.0) and did not
affect vessel conspicuity (Figure 3).

Discussion

This small pilot project investigated the effect of contrast agent on MRA
and the amelioration of artifacts produced by flow and pulsatile movement by
using a centric k-space reordering method. All methods yielded excellent
visualization of small and large vessels, though artifacts hampered
visualization of some vessels in TOF.
Bright
blood visualization of lenticulostriate vessels is extremely difficult at 3T. (3) averaged 3.3 in 100 patients, and 5 or more
were visualized in each patient for each sequence performed in this study,
demonstrating superior performance for MRA at 7T. Variability between patients for the same acquisition was much larger than
variability within a patient for different acquisitions, making it difficult to
make recommendations for targeted 7T imaging with or without contrast. However,
the near elimination of flow and pulsatile artifact with the administration of
contrast, with or without k-space reordering, suggests that contrast enhanced
acquisitions may be recommended when evaluating relatively small pathology in
large vessels. Lastly, Fe-based contrast-enhanced images were qualitatively
better in smaller vessels, though that finding was not supported by the
ratings. 4mg/kg is a small dose of ferumoxytol, and
enhancement might be augmented by administration of 510mg.

Conclusion

All acquisitions were
able to better visualize small vessels at
7T than previously published 3T data. Contrast administration limited flow and pulsatile artifacts, though 3D-centric k-space
reordering had no effect.